Avoid busy patterns on clothing (especially in group sessions) and bright white (cream is ok). Solids are preferable.
Business portraits and family studio sessions long sleeves preferable.
Eyeglasses: If your glasses do not have anti-reflective coating you may want to remove lenses or borrow a pair from store with no lenses to avoid glare.
Transition glasses get dark outside (and sometimes inside too). Please avoid wearing these for outdoor photographs.
Make sure clothes are ironed and wrinkle-free.
Consider your footwear if you want full body poses taken.
Avoid getting sunburned before your session.
If you are very tan avoid wearing clothes that show your tan lines.
You are welcome and encouraged to bring props, such as: toys, instruments, hobby items (skateboards, artwork, etc.) or athletic related materials.